Solid lump fuels such as coal, peat, chips of wood and waste can be burnt in a grate firing, in order to generate steam and possibly eliminate wastes. The grate is disposed in the combustion space either horizontally or at an angle. The grate consists of grate bars or grate plates, in or between which grate slots are provided. Stationary and movable grates are known. In the movable grates, the grate bars or plates are disposed on rotating rollers, or form an endless, moving belt, or are moved by vibration. The fuel is charged onto the grate in the form of a layer. In the case of moving grates, the fuel is transported through the combustion space under the influence of gravity and/or the vibration of the grate bars or the rotation of the grate rollers or the speed of movement of the grate belt, where it burns with the combustion air. The combustion air is supplied to the fuel layer from the bottom through the grate slots (primary air). It is possible to supply part of the combustion air-to the fuel layer also from the top (secondary air). The temperature at the surface of the grate bars and plates during the combustion and in particular during the ignition may reach values between 650 and 950.degree. C., so that a cooling of the grate is required. Such cooling normally is effected by the combustion air. However, there are also known firing grates which are cooled with water. A small part of the ash formed during the combustion drops through the grate slots or through the spaces between the individual grate bar groups and is collected in ash hoppers. In the inclined and/or movable grates, the major part of the ash is dumped at the end of the grate. In the stationary, horizontal grates, the discharge of the ash is effected periodically, e.g. by shaking the grate bars or plates.
From DE-PS 312 287 there is known a water-cooled hollow grate with replaceable bars, where the hollow grate bars have recesses at their ends, in which the connecting pieces provided with flow passages as well as the connecting pieces provided with pipe connections are fixed by pressing devices. The connecting pieces generally connect two grate bars such that the entire grate forms a continuous pipe coil so to speak. The flow of coolant may be designed as desired depending on the arrangement of the connecting pieces provided with pipe connections, and each individual grate bar can be replaced after loosening only two connecting pieces. This construction has the disadvantage that the connecting pieces and abutments are in part disposed in direct vicinity of the combustion space and are therefore subject to a constant wear. In addition, the great distance of the grate bars produces a small grate resistance on the downstream side, which has a negative influence on the course of the combustion.
From DE-PS 340 602 there is known a hollow grate with circulation of water, which has additional air passages that distribute the combustion air over the grate surface. The additional air passages are formed by-the jacket of a water chamber protruding above the grate over its entire length, and below the wided top of the water chamber open into the combustion space. The coolant water flows through the hollow grate bars of the known grate. The grate slots form incisions in the grate bar, around which flows the water contained in the hollow bars. By means of a corresponding line, the circulating water is connected with a feed water tank. By heating the water circulating in the hollow grate, the grate is protected and at the same time the water cooling of the grate is utilized for heating the feed water. This known combustion grate has the disadvantage that it is stationary and can only be operated with low-ash fuels.
DE-AS 1 053 131 discloses a firing grate with divided grate bars, which rest on pipes through which flows water. The grate bars are inserted from the top between the pipes, and are pressed against the pipes with a recess corresponding to the pipe section. This firing grate has the disadvantage that it has a complex technical construction; in technical terms it may be constructed simpler by using jacket pipe walls.
From WO 95/18333 there is finally known a sliding firing grate module for the combustion of waste in commercial plants, which has a plurality of grate steps movable with respect to each other as well as a primary air supply, where its supporting elements as well as its grate consist of hollow parts made of sheet metal, through which flows a liquid cooling medium in the built-in condition. The individual grate steps each consist of a hollow grate plate, the length of which is designed to extend over the entire width of the firing grate to be produced. The grate plate is hollow inside and has at least one connecting port and one discharge port for the supply and discharge of the coolant. Below the grate a pipe extends over the entire length of the grate, which serves as cooling water flow pipe, and from which tube connections lead to each grate plate or grate step or the cooling chambers thereof. Tube connections are provided, which as separate cooling-water recirculations lead back from each grate plate or grate step or the cooling chambers thereof to the control elements for the control of the coolant. The primary air is supplied through holes which extend through the grate plates. This construction has the disadvantage that the combustion air is supplied through the grate bar only partially and does not uniformly aerate the fuel, so that a high thermal load of the grate is impeded.